Superabsorbent polymers in agricultural applications

ABSTRACT

Superabsorbent polymer (SAP) products for use in agricultural applications, and methods of making and using the same are disclosed. Certain of the SAPs include a monomer or a mixture of monomers, other than acrylonitrile, that is graft polymerized onto a starch in the presence of an initiator to form a starch graft copolymer that is cross-linked and the SAP product is isolated.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/604,894, filed Aug. 27, 2004 and entitledALTERNATIVE MONOMERS FOR USE IN PREPARING A SUPERABSORBENT POLYMERPRODUCT, which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a superabsorbent polymer product andto methods of making and applying the superabsorbent polymer product.

BACKGROUND

Superabsorbent polymers (SAPS) are materials that imbibe or absorb atleast 10 times their own weight in aqueous fluid and that retain theimbibed or absorbed aqueous fluid under moderate pressure. The imbibedor absorbed aqueous fluid is taken into the molecular structure of theSAP rather then being contained in pores from which the fluid could beeliminated by squeezing. Some SAPs can absorb up to 1,000 times theirweight in aqueous fluid.

One method of producing a SAP for use in agricultural applicationsinvolves graft polymerizing acrylonitrile onto a starch in the presenceof an initiator, such as a ceric (+4) salt, to form a starch graftcopolymer, and saponifying the nitrile groups with an alkali metal toform a saponificate having alkali carboxylate and carboxamide groups.

Saponification, however, may require expensive machinery and generatesammonia, which can be corrosive, costly to remove, and expensive todispose of. Also, potassium hydroxide (KOH) added during saponificationmakes the saponified starch graft copolymer mixture basic. Acid, e.g.,hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid, isadded to the mixture in order to neutralize the pH of the starch graftcopolymer mixture. If the amount of acid that must be added issignificant, the absorbency of the SAP is reduced. The resulting wastesolutions may also be expensive to dispose of because they includepotassium and ammonium salts and other extraneous salts. Furthermore,acrylonitrile may be hazardous and expensive to dispose of.

BRIEF SUMMARY

The present disclosure presents superabsorbent polymer (SAP) productsfor use in agricultural applications, methods of producing SAP productsand methods of use.

Certain methods of producing SAP products disclosed do not require theuse of acrylonitrile as a monomer and does not require the step ofsaponification. According to one embodiment, the method involves (1)graft polymerizing a monomer, other than acrylonitrile, onto a starch inthe presence of an initiator to form a starch graft copolymer; (2)cross-linking the starch graft copolymer, for example, by adding across-linking agent, such as methylene bis-acrylamide; and (3) isolatingthe starch graft copolymer. The disclosed method may also includeadjusting the pH of the cross-linked starch graft copolymer. Moreover,the method may further include drying the starch graft copolymer, toyield particles that are superabsorbent. The isolation of particles ofsuperabsorbent polymer product may occur by various methods, including,but not limited to, granularization, extrusion, and pelletization.

Certain methods of increasing crop production using a SAP produced bythe above-described method are disclosed. One method involves applyingthe SAP directly to the soil. A second method involves coating a root orseed with the SAP. A third method involves forming a slurry of SAP andwater (or another liquid) and applying the resulting slurry to a plant,root, seed, seedling, or directly to soil into which one of a plant,root, seed, or seedling will be planted.

Certain SAP products for use in agricultural applications are alsodisclosed. The SAP product may include a polysaccharide, such as starchor cellulose, which has a monomer graft polymerized thereto. The monomermay be, for example, acrylic acid or methacrylic acid. The monomer mayalso be acrylamide or methacrylamide. A sulfonic acid, such as2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and vinyl sulfonicacid may also suffice. Moreover, acrylates, such as ethyl acrylate andpotassium acrylate may also be used. Derivatives and mixtures of theabove-listed monomers may also be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting a comparison of sample height according togrowth results described in Table 1;

FIG. 2 is a graph depicting a comparison of sample width according togrowth results described in Table 1; and

FIG. 3 is a graph depicting a comparison of sample mass according togrowth results described in Table 1.

DETAILED DESCRIPTION

Those skilled in the art will recognize that the methods andcompositions disclosed herein may be practiced without one or more ofthe specific details described, or with other methods, components,materials, etc. In some cases, well-known materials, components ormethod steps are not shown or described in detail. Furthermore, thedescribed method steps, compositions, etc., may be combined in anysuitable manner in one or more embodiments. It will also be readilyunderstood that the methods and compositions of the embodiments asgenerally described herein could be arranged and designed in a widevariety of different configurations.

The order of the steps or actions of the methods described in connectionwith the embodiments disclosed may be changed as would be apparent tothose skilled in the art. Thus, any order in the detailed description isfor illustrative purposes only and is not meant to imply a requiredorder.

One embodiment of a method of making a superabsorbent polymer (SAP) foruse in large-scale agricultural applications comprises (1) graftpolymerizing a monomer onto a starch in the presence of an initiator toform a starch graft copolymer; (2) cross-linking the starch graftcopolymer, for example, by adding a cross-linking agent, such asmethylene bis-acrylamide to cross-link the starch graft copolymer; (3)adjusting the pH of the cross-linked starch graft copolymer, such asneutralization; (4) isolating the cross-linked starch graft copolymer;and (5) drying the cross-linked starch graft copolymer.

Exemplary monomers for use in the above-described method include acrylicacid or methacrylic acid. Exemplary monomers may also include acrylamideor methacrylamide. Sulfonic acids, such as2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and vinyl sulfonicacid may also be used. Moreover, acrylates, such as ethyl acrylate andpotassium acrylate may also be used. Derivatives and mixtures of theabove-listed monomers may also be desirable.

For example, in some applications it may be desirable to use acrylicacid as the monomer. In other applications it may be desirable to use amixture of acrylic acid and acrylamide to be graft polymerized onto astarch. In other alternative applications, it may be desirable to use2-acrylamido-2-methyl-propanesulfonic acid.

In applications using acrylic acid, the addition of acrylamide theretohelps induce graft polymerization and adds to absorbency of the SAP. Byway of example, the ratio by weight of acrylic acid to acrylamide may beabout 2:1. Alternatively, the ratio of acrylic acid to acrylamide mayalso range up to a ratio of 9:1 and beyond. Because acrylamide isconsidered a neurotoxin, it may be desirable to reduce the relativeamount of acrylamide to acrylic acid, while using enough to help inducegraft polymerization of acrylic acid.

In alternative applications, acrylic acid may graft polymerize onto astarch or other polysaccharide without the assistance of acrylamide. Forexample, acrylic acid may polymerize when placed under heat and/orpressure. Polymerization without the addition of acrylamide may beaccomplished, for example, in a heated screw extruder, such as a singlescrew or a double screw.

The starches used in the above-described method include starches,flours, and meals. More specifically, exemplary starches include nativestarches (e.g., corn starch (Pure Food Powder, manufactured by A.E.Staley), waxy maize starch (Waxy 7350, manufactured by A.E. Staley),wheat starch (Midsol 50, manufactured by Midwest Grain Products), potatostarch (Avebe, manufactured by A.E. Staley)), dextrin starches (e.g.,Stadex 9, manufactured by A.E. Staley), dextran starches (e.g., Grade2P, manufactured by Pharmachem Corp.), corn meal, peeled yucca root,unpeeled yucca root, oat flour, banana flour, and tapioca flour. Thestarch may be gelatinized to provide optimal absorbency. An exemplarystarch is gelatinized cornstarch. Furthermore, according to oneembodiment, the weight ratio of the starch to the monomer is in therange of between about 1:1 and about 1:6.

In alternative embodiments, other polysaccharides, such as cellulose,may be used instead of starch. Accordingly, the monomers heretoforedescribed may be graft polymerized onto cellulose for purposes ofagricultural applications.

The monomer may be graft polymerized onto a starch in the presence of aninitiator. Exemplary initiators for use in the above-described methodinclude: cerium (+4) salts, such as ceric ammonium nitrate; ammoniumpersulfate; sodium persulfate; potassium persulfate; ferrous peroxide;ferrous ammonium sulfate-hydrogen peroxide; L-ascorbic acid; andpotassium permanganate-ascorbic acid. Other suitable initiators known tothose skilled in the art may be used, such as alternative persulfatesand peroxides, as well as vanadium, manganese, etc. The amount ofinitiator used may vary based on the chosen initiator, the selectedmonomer, and the chosen starch. Some initiators, e.g., persulfates, mayrequire the presence of heat. The initiator may be added in a single ormultiple steps, and multiple initiators may be used.

A cross-linking agent may be added to the mixture to form a cross-linkedstarch graft copolymer. It may be desirable for the starch graftcopolymer to be cross-linked if it dissolves in aqueous fluids previousto being cross-linked. Cross-linking is one method to permit the starchgraft copolymer to absorb aqueous fluids without dissolving. However,the amount of cross-linking agent added is typically indirectlyproportional to the absorbency of the resulting SAP product. Exemplarycross-linking agents include: glycerides; diepoxides; diglycidyls;cyclohexadiamide; methylene bis-acrylamide; bis-hydroxyalkylamides, suchas bis-hydroxypropyl adipamide; formaldehydes, such as urea-formaldehydeand melamine-formaldehyde resins; isocyanates including di- ortri-isocyanates; epoxy resins, typically in the presence of a basecatalyst; and derivatives and mixtures thereof.

Alternative methods of cross-linking may also be employed. For example,a solid SAP product may be cross-linked through irradiation, such asexposure to gamma or x-ray electromagnetic radiation, or to an electronbeam and the like. Irradiation facilitates cross-linking of the starchgraft copolymer by creating free radicals in the copolymer chain. Insome applications, after irradiation an annealing or melting process maybe used in re-forming the cross-linked copolymer chains. Furthermore, itmay be desirable to perform the irradiation process in an atmosphererelatively free of oxygen.

Although the addition of cross-linking agents may be desirable in theproduction of SAPs, self-cross-linking copolymers may also be used. In aself-cross-linking copolymer, either a single self-reactive functionalgroup or multiple self-reactive functional groups or multipleco-reactive functional groups are incorporated into the mixture. Oneexemplary co-reactive functional group is a copolymer of acrylic acidand glycidyl methacrylate.

Once a cross-linked starch graft copolymer is formed, the pH of thecross-linked starch graft copolymer may be adjusted to a desired valuefor the particular agricultural application. For example, thecross-linked starch graft copolymer may be neutralized to convert thecarboxyl groups to potassium salts. Alternative pH values may bedesirable depending upon the type of soil and the type of crop theresulting SAPs will be applied to. The resulting pH for mostagricultural applications typically will range from about 6.0 to about8.0. The desired pH may be greater or less than this range depending onthe requirements for the particular agricultural application.

Alternatively, in some embodiments, pH adjustment of the starch graftcopolymer may occur prior to cross-linking. In contrast to somealternative methods which require saponification, the step of pHadjustment/neutralization may be significantly faster, easier, and lessexpensive compared to saponification. Furthermore, adjusting the pH doesnot necessarily produce corrosive and dangerous reaction by-productssuch as ammonia. Exemplary solvents that may be used to effect pHadjustment include potassium hydroxide, potassium methoxide, or amixture thereof, any of which may optionally be diluted in methanol orother solvents.

In alternative embodiments, pH adjustment may not be necessary. Forinstance, if potassium acrylate were used as the monomer in lieu ofacrylic acid, the resulting product may already be within an acceptablepH range.

In one embodiment, the resulting pH adjusted, cross-linked starch graftcopolymer may then be isolated. One exemplary method of isolationinvolves simply drying the cross-linked starch graft copolymer, such as,for example, on a heated drum or via air-drying. The dried SAP productmay then be pelletized according to pelletization methods known to thosehaving skill in the art.

Compared to some alternative methods of producing SAPs which require thestep of saponification, the method described herein provides apH-adjusted, cross-linked starch graft copolymer reaction mass havingvery little extraneous salt. Consequently, isolation can be effectedthrough the step of drying the SAP product in an alcohol-freeenvironment. In contrast, methods that require saponification result instarch graft copolymers having a significant amount of extraneous saltand ammonia and thus must be treated with methanol. The use of methanolmay add significantly to the cost of producing the SAP product becausemethanol disposal can be expensive.

In another embodiment, the step of isolating the starch graft copolymerinvolves extruding the cross-linked starch graft copolymer such asthrough a heated screw to form granules of SAP product. To minimizere-agglomeration of the granules, the granules may be coated with adusting agent that decreases their propensity to stick together.Exemplary dusting agents include cellulose, clay, starch, flour, andother natural or synthetic polymers that prevent the granules fromsticking together. Alternatively, the granules may be lightly sprayedwith methanol to prevent them from sticking together, and/or theextrusion can be performed under high pressure.

Yet another exemplary method of isolating the starch graft copolymerinvolves precipitating the pH-adjusted, cross-linked starch graftcopolymer using water-miscible solvents such as alcohols, e.g.,methanol, ethanol, propanol, and isopropanol. Immersing the cross-linkedstarch graft copolymer in alcohol may cause the alkali starch graftcopolymer to precipitate into particles that are later screened to thedesired size after drying. The alcohol removes the water and extraneoussalts from the cross-linked starch graft copolymer.

Another exemplary implementation of this method of precipitationinvolves blending sufficient methanol into the pH-adjusted, cross-linkedstarch graft copolymer to achieve a smooth dispersion. The smoothdispersion may then be pumped into a precipitation tank, which mayinclude a stirring system that can vigorously mix the methanol whilepumping in the smooth cross-linked starch graft copolymer dispersion.Once mixed, the resulting methanol and cross-linked starch graftcopolymer particles may be collected by decanting or washing withmethanol or centrifuged and collected, then dried to a moisture level ofbetween about 1 percent and about 20 percent.

A third implementation of the isolation step through precipitation withmethanol involves wetting the surface of the cross-linked starch graftcopolymer with a small amount of methanol and then chopping thecross-linked starch graft copolymer into larger “chunks” that will notre-adhere to one another. Once the surface of the cross-linked starchgraft copolymer has been wetted with methanol, the resulting material isslippery to the touch and is no longer sticky. This effect may beachieved by using a compositional ratio of between about one part andabout two parts of methanol per one part of solid.

Once the methanol has been added, the cross-linked starch graftcopolymer may be pumped through an in-line chopper to form chunks havinga diameter of less than one inch or, alternatively, hand-chopped withscissors. The resulting mixture is then fed into a tank or Waringblender that has between about 1.5 gallons and about 2.0 gallons ofadditional methanol per pound of cross-linked starch graft copolymer. Insome embodiments, the cross-linked starch graft copolymer may be subjectto a pulverizer, such as an in-line mixer or disintegrator which breaksthe mass into smaller pieces as desired for the particular application.The methanol in the larger tank may be agitated with a Cowles dissolveror other mixer capable of achieving high speeds.

A fourth implementation of the isolation step through precipitation withmethanol involves pre-forming the particle size before the methanolprecipitation step. The use of dies to form strands or rods havingdifferent shapes and diameters can greatly improve the particle sizeformation process. This fourth implementation offers enhanced control ofthe final particle size. The cross-linked starch graft copolymer(neutralized or unneutralized) may be forced through a die plate havingholes of varying diameter (e.g., about 1/16 inch to more than ¼ inch)and varying shape (e.g., round, star, ribbon, etc.).

Methods of forcing the cross-linked starch graft copolymer through thedie plate include using a hand-operated plunger, screw-feeding,auguring, pumping, and any other commonly known method. The resultingstrands or rods may be placed into the precipitation tank without anyfurther addition of methanol as a premixing agent. The strands or rodsmay be treated to prevent them from sticking together by, for example,wetting or spraying the strands or rods with methanol or dusting themwith a dusting agent, such as, for example, cellulose, clay, starch,flour, or other natural or synthetic polymers. The resulting strands orrods may be precipitated with agitated methanol, removed from the tank,and dried.

Another step in the method of preparing a SAP includes forming theisolated, cross-linked starch graft copolymer into the desired size ofparticles and drying. The SAP product may have a particle size of lessthan about 200 mesh. The desirable particle size may depend on thespecific agricultural application intended. In one embodiment foragricultural applications that deposit the starch graft copolymerdirectly into the soil, the particle size may be less than 50 mesh, moreparticularly between about 5 mesh and 50 mesh, or between about 5 meshand 25 mesh, or between about 8 mesh and about 25 mesh. This particlesize is typically compatible with commercially available granularapplicators in the industry. To broadcast or meter the absorbentparticles through most existing application equipment, an about 8 meshto about 25 mesh SAP product having a density of between about 30 poundsand about 35 pounds per cubic foot may be used.

Other agricultural applications, such as seed coating and root dipping,may use a finer particle size. For seed coating, the desired particlesize may be between about 75 mesh and about 300 mesh, such as about 200mesh. For root dipping, the desired particle size may be between about30 mesh and about 100 mesh, such as about 50 mesh.

Alternatively, the cross-linked cross-linked starch graft copolymerproduct may be mixed with a solvent, such as water, to form a slurry.The resulting slurry may be applied to an agricultural medium such as aplant, root, seed, seedling, or directly to soil into which one of aplant, root, seed, or seedling will be planted.

One exemplary method by which the desired size of particles may beformed involves converting the cross-linked starch graft copolymer intorod-shaped forms and drying the forms to the desired particle size. Dieselection typically dictates the size and shape of the rod-shaped forms.The diameter of the rods is controlled by drilling holes in the endplate, such as 1/16-inch to ¼-inch in diameter. For example, the diewould be a plate that has been drilled or formed to contain holes of theselected size and shape.

Following extrusion from the die, the rod-shaped forms may be lightlycoated with a dusting agent that decreases their propensity to sticktogether and reduces their tackiness. Exemplary dusting agents includecellulose, clay, starch, flour, and other natural or synthetic polymersthat prevent the rods from sticking together. Alternatively, the rodsmay be lightly sprayed with methanol, and/or they may be extruded fromthe die under pressure. The coated particles are then dried. Exemplarydrying methods include air-drying or oven-drying. Following drying, theparticles may be screened to the appropriate size.

In another exemplary method by which the desired size particles may beformed, the cross-linked starch graft copolymer may be ground to a finepowder and then formed into pellets of the desired size. Pelletizing iscommon in the polymer industry and is known to those of skill in theart. As described above, the resulting pellets may be lightly coatedwith a dusting agent that decreases their propensity to stick togetherand reduces their tackiness.

The SAP product made by the methods described herein may also be coloredusing any coloring method known to one of skill in the art, including,but not limited to, adding fertilizers and/or charcoal. Also, afertilizer or micronutrient may be added to the SAP product. Thefertilizer or micronutrient may be added once the granular SAP productis formed or at any stage during processing.

The agricultural application of SAPs made by the above-described methodsmay result in earlier seed germination and/or blooming, decreasedirrigation requirements, increased propagation, increased crop growth,increased crop production, and decreased soil crusting. Thus SAPs madeby the methods disclosed herein are desirable for forming and using aSAP in large-scale agricultural applications.

The following Examples 1-3 demonstrate exemplary procedures used to forma SAP product using the method(s) described herein:

EXAMPLE 1

Deionized water (2,000 ml) was added to cornstarch (200 g; Cargill GelInstant 12030, manufactured by Cargill Food and Pharma Specialties, Inc.of Cedar Rapids, Iowa) in a 3-liter resin kettle. The combination wasmixed until a uniform mixture was formed. Acrylic acid (200 g; 99%purity; City Chemical, LLC of West Haven, Conn.) was added to the cooledmixture and the resulting mixture was stirred for approximately fiveminutes. Next, acrylamide (100 g; 99% purity; City Chemical, LLC of WestHaven, Conn.) was added to the mixture, and the resulting mixture wasstirred for approximately five minutes. Then methylene bis-acrylamide(0.5 g dissolved in 50 ml of deionized water; Molecular Grade; 99%purity; manufactured by Promega Corporation of Madison, Wis.) was addedto the mixture, and the resulting mixture was stirred for approximatelyfive minutes. Lastly, ammonium persulfate (0.5 g dissolved in 50 ml ofdeionized water; Molecular Grade; 99% purity; manufactured by CascadeColumbia Distribution Co. of Sherwood, Oreg.) was added to the mixtureand the resulting mixture was stirred while being heated toapproximately 170° F. The mixture was held at that temperature andstirred for approximately 15 minutes. The resulting white, viscous masshad a pH of 3.7, and a nitrogen test of a small sample of the viscousmass showed a nitrogen content of 3.58%.

Because the resulting viscous mass was acidic, the mixture wasneutralized by titration with 45% potassium hydroxide (KOH) at roomtemperature. Titration continued until a pH of 7.0 was reached, whichrequired addition of between about 160 g and 170 g of 45% KOH.

The cross-linked SAP product was then isolated by adding the neutral pHreaction mass to several gallons of methanol. The resulting cross-linkedSAP product was dried in a tumble dryer such that a white, granular SAPproduct having a density of 6.6 grams per cubic inch, and a moisturecontent of 9.1% was formed. A nitrogen test of the SAP product showed anitrogen content of approximately 3.19%. The SAP product exhibited theability to imbibe or absorb between about 400 and about 500 times itsweight in aqueous fluid and to retain the imbibed or absorbed aqueousfluid under moderate pressure.

EXAMPLE 2

Deionized water (2,000 ml) was added to cornstarch (200 g; Corn Products#3005, Industrial Starch (pearl starch), manufactured by CPCInternational, Inc. of Westchester, Ill.) in a 3-liter resin kettle. Thecombination was mixed until a uniform mixture was formed. The mixturewas then heated to between about 185° F. and about 190° F. using aheating jacket. The mixture was maintained at this temperature forapproximately 30 minutes, at which time the heating jacket was turnedoff and the mixture was allowed to cool to 150° F.

Acrylic acid (200 g; 99% purity; City Chemical, LLC of West Haven,Conn.) was added to the cooled mixture and the resulting mixture wasstirred for approximately five minutes. Next, acrylamide (100 g; 99%purity; City Chemical, LLC of West Haven, Conn.) was added to themixture, and the resulting mixture was stirred for approximately fiveminutes. Then methylene bis-acrylamide (0.5 g dissolved in 50 ml ofdeionized water; Molecular Grade; 99% purity; manufactured by PromegaCorporation of Madison, Wis.) was added to the mixture, and theresulting mixture was stirred for approximately five minutes. Lastly,ammonium persulfate (0.5 g dissolved in 50 ml of deionized water;Molecular Grade; 99% purity; manufactured by Cascade ColumbiaDistribution Co. of Sherwood, Oreg.) was added to the mixture and theresulting mixture was stirred while being heated to approximately 170°F. The mixture was held at that temperature and stirred forapproximately 15 minutes. The resulting white, viscous mass had a pH of3.7, and a nitrogen test of a small sample of the viscous mass showed anitrogen content of 3.58%.

Because the resulting viscous mass was acidic, the mixture wasneutralized by titration with 45% potassium hydroxide (KOH) at roomtemperature. Titration continued until a pH of 7.0 was reached, whichrequired addition of between about 160 g and 170 g of 45% KOH.

The cross-linked SAP product was then isolated by adding the neutral pHreaction mass to several gallons of methanol. The resulting cross-linkedSAP product was dried in a tumble dryer such that a white, granular SAPproduct was formed. The SAP product exhibited the ability to imbibe orabsorb between about 400 and about 500 times its weight in aqueous fluidand to retain the imbibed or absorbed aqueous fluid under moderatepressure.

EXAMPLE 3

Deionized water (2,000 ml) was added to pregelatinized yellow corn flour(200 g; #01965-00, manufactured by Cargill Dry Corn Ingredients, Inc. ofParis, Ill.) in a 3-liter resin kettle. The combination was mixed untila uniform mixture was formed. Acrylic acid (200 g; 99% purity; CityChemical, LLC of West Haven, Conn.) was added to the cooled mixture andthe resulting mixture was stirred for approximately five minutes. Next,acrylamide (100 g; 99% purity; City Chemical, LLC of West Haven, Conn.)was added to the mixture, and the resulting mixture was stirred forapproximately five minutes. Then methylene bis-acrylamide (0.5 gdissolved in 50 ml of deionized water; Molecular Grade; 99% purity;manufactured by Promega Corporation of Madison, Wis.) was added to themixture, and the resulting mixture was stirred for approximately fiveminutes. Lastly, ammonium persulfate (0.5 g dissolved in 50 ml ofdeionized water; Molecular Grade; 99% purity; manufactured by CascadeColumbia Distribution Co. of Sherwood, Oreg.) was added to the mixtureand the resulting mixture was stirred while being heated toapproximately 170° F. The mixture was held at that temperature andstirred for approximately 15 minutes. The resulting white, viscous masshad a pH of 3.7, and a nitrogen test of a small sample of the viscousmass showed a nitrogen content of 3.58%.

Because the resulting viscous mass was acidic, the mixture wasneutralized by titration with 45% potassium hydroxide (KOH) at roomtemperature. Titration continued until a pH of 7.0 was reached, whichrequired addition of between about 160 g and 170 g of 45% KOH.

The cross-linked SAP product was then isolated by adding the neutral pHreaction mass to several gallons of methanol. The resulting cross-linkedSAP product was dried in a tumble dryer such that a white, granular SAPproduct was formed. The SAP product exhibited the ability to imbibe orabsorb between about 400 and about 500 times its weight in aqueous fluidand to retain the imbibed or absorbed aqueous fluid under moderatepressure.

The following Examples 4 and 5 are hypothetical examples thatdemonstrate exemplary procedures that may be used to form a SAP productusing the method(s) described herein. While Examples 4 and 5 arehypothetical in nature they are based upon actual experimental designsthat have been tested and/or contemplated.

EXAMPLE 4

Deionized water (2,000 ml) is added to cornstarch (200 g) in a 3-literresin kettle. The combination is mixed until a uniform mixture isformed. Acrylic acid (200 g; 99% purity) is added to the cooled mixtureand the resulting mixture is stirred for approximately five minutes.Next, acrylamide (100 g; 99% purity) is added to the mixture, and theresulting mixture is stirred for approximately five minutes. Thenmethylene bis-acrylamide (0.5 g dissolved in 50 ml of deionized water;Molecular Grade; 99% purity) is added to the mixture, and the resultingmixture is stirred for approximately five minutes. Lastly, ammoniumpersulfate (0.5 g dissolved in 50 ml of deionized water; MolecularGrade; 99% purity) is added to the mixture and the resulting mixture isstirred while being heated to approximately 170° F. The mixture is heldat that temperature and stirred for approximately 15 minutes.

The resulting mass is neutralized by titration with 45% potassiumhydroxide (KOH) at room temperature. Titration continues until a pH of7.0 is reached. The cross-linked SAP product is then dried in a tumbledryer.

EXAMPLE 5

Deionized water (2,000 ml) is added to cornstarch (200 g) in a 3-literresin kettle. The combination is mixed until a uniform mixture isformed. Acrylic acid (200 g; 99% purity) is added to the cooled mixtureand the resulting mixture is stirred for approximately five minutes.Next, acrylamide (100 g; 99% purity) is added to the mixture, and theresulting mixture is stirred for approximately five minutes. Thenmethylene bis-acrylamide (0.5 g dissolved in 50 ml of deionized water;Molecular Grade; 99% purity) is added to the mixture, and the resultingmixture is stirred for approximately five minutes. Lastly, ammoniumpersulfate (0.5 g dissolved in 50 ml of deionized water; MolecularGrade; 99% purity) is added to the mixture and the resulting mixture isstirred while being heated to approximately 170° F. The mixture is heldat that temperature and stirred for approximately 15 minutes.

The resulting mass is neutralized by titration with 45% potassiumhydroxide (KOH) at room temperature. Titration continues until a pH of7.0 is reached. The neutralized cross-linked starch graft copolymer isthen screw-fed through a die plate having holes of varying diameter(between 1/16 inch to ¼ inch). The resulting strands are dusted withcellulose as a dusting agent, to prevent the strands from stickingtogether. The resulting strands are then dried in a tumble dryer.

Experimental Comparison

The effectiveness of the SAP product formed using the method describedin Example 1 was tested and analyzed in comparison to variousalternative SAP products having varying particle sizes and in comparisonto control subjects.

The general procedure for the testing was as follows. Eight one-gallonplastic pots having drainage holes were obtained. Six of the pots(Samples B-E, G, and H) were filled with a thoroughly combined mixtureof 10 g of the selected SAP product and approximately 0.5 gallon ofsand; two of the pots (Samples A and F) were control pots, which werefilled with plain, untreated sand. The assignment of pots (A through H)occurred randomly. Sand, rather than dirt, was chosen as the growingmedium because sand provides no nutrients for growing plants. SamplesB-E, G, and H were formed as follows:

-   -   Sample B included an alternative SAP product having a particle        size of about 10 to about 20 mesh and made using acrylonitrile        as the monomer and yellow corn flour as the starch;    -   Sample C included the SAP product formed by the method described        in Example 1 and having a particle size of about 8 to about 16        mesh;    -   Sample D included another alternative SAP product having a        particle size of about 8 mesh and made using acrylonitrile as        the monomer and cornstarch as the starch;    -   Sample E included yet another alternative SAP product having a        particle size of about 10 to about 20 mesh and using        acrylonitrile as the monomer and a 50/50 mixture of yellow corn        flour and cornstarch as the starch;    -   Sample G included still another alternative SAP product having a        particle size of greater than about 8 mesh and made using        acrylonitrile as the monomer and cornstarch as the starch; and    -   Sample H included the SAP product formed by the method described        in Example 1 and having a particle size of between about 20 and        about 40 mesh.

One six-inch-high geranium plant was planted in each of the filled pots,such that the final sand level in each pot was approximately one-inchbelow the rim of the pot. Approximately two liters of water were addedto each pot; 24 hours later, another approximately two liters of waterwere added to each pot.

The pots were then placed in a plastic pool that was positioned under afluorescent light source, such that the light source was approximately14 inches above the tops of the geranium plants. The plastic pool wasslightly rotated on a daily basis to ensure that each geranium plantreceived the same amount of light. Once per week, the plants wererearranged in the pool, to further ensure that each geranium plantreceived the same amount of light. The geranium plants received noadditional water and were allowed to grow for 65 days. At the end of 65days, the geranium plants were harvested and the following informationwas gathered: TABLE 1 Growth Results Height^(¥) Width^(£) Sample(inches) (inches) Mass (g)* Physical Description A 4 3 24.3 Wilted,slightly yellowing, small leaves; little observable growth; minimal rootgrowth B 10 9 55.7 Healthy; observable new growth; large, green leaves;tall plant C 7 11 65.3 Healthy; observable new growth; large, greenleaves; bushy plant D 8 7 45.1 Relatively healthy; observable newgrowth; large, green leaves on top and some yellow leaves on bottom;tall plant E 9 12 67.6 Healthy, observable new growth; large, greenleaves; bushy plant F 4 3 22.2 Wilted, slightly yellowing, small leaves;little observable growth; minimal root growth G 10 12 53.4 Healthy;observable new growth; large, green-and- yellow leaves; tall and bushyplant H 9 10 65.3 Healthy; observable new growth; large, green leaves;tall and bushy plant^(¥)Height of the plant is measured from the top of the sand to the topof the highest point of the plant.^(£)Width of the plant is measured from the widest points on either sideof the plant.*Mass includes the entire plant (roots, stem, and leaves).

FIG. 1 is a graph representing the comparison of sample height, wherethe x-axis is each sample and the y-axis is the height in inches. Theheight of each plant is measured from the top of the sand to the top ofthe highest point of the plant. FIG. 2 is a graph representing thecomparison of sample width, where the x-axis is the sample identity andthe y-axis is the width of the sample in inches. The width of each plantis measured from the widest points on either side of the plant. FIG. 3is a graph representing the comparison of sample mass, where the x-axisis the sample identity and the y-axis is the mass of each sample ingrams. The mass includes the mass of the entire plant (roots, stem, andleaves).

FIGS. 1 through 3 show that Sample E, which included an alternative SAPproduct having a particle size of about 10 to about 20 mesh and usingacrylonitrile as the monomer and a 50/50 mixture of yellow corn flourand cornstarch as the starch, had the greatest overall mass (67.6 g).However, Samples C and H, which both included the SAP product formed bythe method described in Example 1, tied for the second greatest overallmass. Interestingly, there was no noticeable difference in overall massbased on the varying particle sizes used for Samples C and H. Mostimportantly, both Samples C and H showed significant growth as comparedto the control samples (A and F).

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments.Furthermore, the methods disclosed herein comprise one or more steps oractions for performing the described method. The method steps and/oractions may be interchanged with one another. In other words, unless aspecific order of steps or actions is required for proper operation ofthe embodiment, the order and/or use of specific steps and/or actionsmay be modified without departing from the scope of the invention asclaimed hereinafter.

1. A superabsorbent polymer product, comprising: a polysaccharide; and a monomer that is graft polymerized onto the polysaccharide in the presence of an initiator, forming a polysaccharide graft polymer; wherein the polysaccharide graft polymer is cross-linked and further isolated in an alcohol-free environment.
 2. The superabsorbent polymer product of claim 1, wherein the monomer is at least one of the following: acrylic acid, acrylamide, methacrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate, and derivatives and mixtures thereof.
 3. The superabsorbent polymer product of claim 1, wherein the monomer is a mixture of acrylic acid and acrylamide.
 4. The superabsorbent polymer product of claim 3, wherein the mixture is about two or more parts acrylic acid to about one part acrylamide by weight.
 5. The superabsorbent polymer product of claim 1, wherein the monomer is 2-acrylamido-2-methyl-propanesulfonic acid and derivatives thereof.
 6. The superabsorbent polymer product of claim 1, wherein the polysaccharide is a starch.
 7. The superabsorbent polymer product of claim 6, wherein the starch is at least one of the following: corn starch, waxy maize starch, wheat starch, potato starch, dextrin starch, dextran starch, corn meal, yucca root, oat flour, banana flour, and tapioca flour.
 8. The superabsorbent polymer product of claim 6, wherein the weight ratio of starch to monomer is in the range of between about 1:1 and about 1:6.
 9. The superabsorbent polymer product of claim 1, wherein the initiator is ammonium persulfate.
 10. The superabsorbent polymer product of claim 1, wherein the initiator is at least one of the following: cerium (+4) salt, ammonium persulfate, sodium persulfate, potassium persulfate, ferrous peroxide, ferrous ammonium sulfate-hydrogen peroxide, L-ascorbic acid, and potassium permanganate-ascorbic acid.
 11. A method for preparing a superabsorbent polymer, comprising: graft polymerizing a monomer onto a starch in the presence of an initiator to form a starch graft copolymer; cross-linking the starch graft copolymer; and isolating the starch graft copolymer in an alcohol-free environment.
 12. The method of claim 11, further comprising drying the starch graft copolymer.
 13. The method of claim 11, further comprising adjusting a pH of the starch graft copolymer.
 14. The method of claim 13, wherein the pH is adjusted to within a range of about 6.0 to about 8.0.
 15. The method of claim 11, wherein isolating the starch graft copolymer comprises granularizing the starch graft copolymer.
 16. The method of claim 11, wherein isolating the starch graft copolymer comprises extruding the starch graft copolymer.
 17. The method of claim 11, wherein isolating the starch graft copolymer comprises pelletizing the starch graft copolymer.
 18. The method of claim 11, wherein isolating the starch graft copolymer comprises drying the starch graft copolymer.
 19. The method of claim 11, wherein the monomer is a mixture of acrylic acid and acrylamide.
 20. The method of claim 11, wherein the monomer is at least one of the following: acrylic acid, acrylamide, methacrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate, and derivatives and mixtures thereof.
 21. The method of claim 11, wherein cross-linking the starch graft copolymer comprises adding a cross-linking agent.
 22. A method for preparing a superabsorbent polymer product for agricultural applications, comprising: graft polymerizing a mixture of acrylic acid and acrylamide onto a starch in the presence of an initiator to form a starch graft copolymer; adding a cross-linking agent to form a cross-linked starch graft copolymer; adjusting a pH of the cross-linked starch graft copolymer to within a pH range of about 6.0 and about 8.0; and isolating the superabsorbent polymer product through drying or extruding the cross-linked starch graft copolymer.
 23. A method of using a superabsorbent polymer product, comprising: obtaining a cross-linked starch graft copolymer product isolated in an alcohol-free environment; and applying the cross-linked starch graft copolymer product to an agricultural medium to increase plant growth.
 24. The method of claim 23, wherein applying the cross-linked starch graft copolymer product to an agricultural medium comprises applying the cross-linked starch graft copolymer to soil.
 25. The method of claim 23, wherein applying the cross-linked starch graft copolymer product to an agricultural medium comprises coating a seed with the cross-linked starch graft copolymer.
 26. The method of claim 23, wherein applying the cross-linked starch graft copolymer product to an agricultural medium comprises coating a plant root with granulated cross-linked starch graft copolymer.
 27. The method of claim 23, further comprising forming a slurry of the cross-linked starch graft copolymer product with a solvent, wherein applying the cross-linked starch graft copolymer product to an agricultural medium comprises applying the slurry to at least one of the following: a plant, a root, a seed, a seedling, and soil.
 28. The method of claim 23, wherein the cross-linked starch graft copolymer product comprises a mixture of acrylic acid and acrylamide that is graft polymerized onto a starch. 